   
图1 外源Spd对盐胁迫下黄瓜幼苗叶片游离态多胺含量的影响
Fig.1 Effect of exogenous Spd on freepolyamines content in cucumber seedlings under salt stress.
2.3叶片总RNA的提取
如图2，黄瓜叶片的总RNA经琼脂糖凝胶电泳检测呈现出3条清晰可区分的条带，表明RNA未降解。经过生物分光光度计(biophotometer plus)检测，RNA样品A260/A280的比值在1.8~2.0之间，表明RNA纯度较高。
图2 黄瓜叶片总RNA琼脂糖凝胶电泳检测结果
Fig.2 Result of agarose gel electrophoresisof total RNA extracted from cucumber leaf
2.4叶片多胺合成酶基因表达
如图3所示，在正常栽培条件下，外源喷施Spd后，adc、spds基因表达有所下调，而odc、samdc基因没有明显变化；在盐胁迫下，多胺合成酶adc、odc、samdc、spds基因相比对照都有不同程度的上调，明显高于对照；盐胁迫下喷施Spd后，进一步上调了samdc基因的表达，下调了adc、odc、spds基因的表达。
图3 外源Spd对盐胁迫下黄瓜幼苗叶片多胺合成酶基因表达的影响
Fig.3 Effect of exogenous Spd on polyamine synthasegene in cucumber leaves under salt stress.
3 讨论
盐胁迫扰乱了植物体内离子分布和水分平衡，使植物发生离子毒害和渗透胁迫，进而导致次生代谢紊乱，如营养失衡和氧化胁迫[2]。盐胁迫使植株叶片失绿和提早衰亡，降低植物绝对/相对生长速率和生物量的积累，植物最普遍和最显著的变化就是生长受到抑制[21]。本试验中，75mmol·L-1NaCl胁迫抑制了黄瓜幼苗的生长，显著降低了株高、茎粗和生物量的积累；而盐胁迫下外源喷施Spd，植株株高、茎粗和干鲜重下降缓慢，明显减缓了盐胁迫对植株生长的抑制。
多胺是生物代谢过程中产生的一类具有生物活性的低分子量脂肪族含氮碱，在逆境胁迫下发挥着重要的作用，如稳定生物膜[22]、清除活性氧自由基[23,24]、稳定生物大分子[25]、影响跨膜质子电化学梯度[26]、参与细胞信号转导[27]等。论文大全，盐胁迫。植物在响应逆境胁迫时，体内多胺合成酶活性发生改变，多胺浓度迅速发生变化。一般研究认为盐胁迫下高含量的Spd和Spm积累有利于缓解高等植物盐胁迫伤害，盐胁迫下玉米能积累较高浓度的Spd和Spm[14]，水稻抗盐品种积累较多的Spd和Spm,而盐敏感品种体内Spd和Spm的含量较低[28]；游离态Put的积累是大麦幼苗耐盐性的负影响因子，提高游离态Put向游离态Spd 、Spm以及结合态多胺转变能缓解盐胁迫伤害及毒害[29]；细胞水平上的研究表明，Put向Spd的转化有利于提高向日葵愈伤组织的耐盐性[30]。本试验中，75mmol·L-1NaCl胁迫下，黄瓜叶片游离态Put、Spd和Spm含量迅速上升，盐胁迫下喷施Spd进一步促进了游离态Spd和Spm含量的增加，抑制了游离态Put含量的升高，缓解了盐胁迫对植株生长的抑制。论文大全，盐胁迫。
adc、odc、samdc、spds是编码多胺合成关键酶的基因，非生物逆境下多胺含量的变化与多胺合成酶基因表达密切相关。研究表明，盐胁迫下苹果愈伤组织中adc基因表达引起ADC活性增强，Put含量增加[31]；通过基因工程的方法使梨树过量表达spds基因，盐胁迫下植株Spd含量增加，MDA、H2O2含量明显降低，SOD、APX、MDHAR、GR活性显著上升，抗氧化能力增强、耐盐性提高[32]。在过表达samdc基因的水稻中，发现Spd和Spm含量增加，植株的盐胁迫耐性增强[33]。研究发现，盐胁迫下黄瓜ADC和ODC活性显著升高，促进了黄瓜植株Put的合成[20]。本试验中盐胁迫下adc和odc基因表达上调，与酶活性升高相一致，与游离态Put含量显著升高相对应；而盐胁迫下喷施Spd后，adc和odc基因表达下降，与游离态Put含量下降一致。论文大全，盐胁迫。盐胁迫同时诱导了samdc和spds基因的表达，外源Spd进一步上调了samdc基因的表达，部分下调了spds基因的表达，但是spds基因相对表达水平仍高于对照，说明盐胁迫喷施Spd仍能促进内源Spd的合成，外源Spd可能通过诱导samdc基因表达，加速了植物中氨丙基的供应，有利于游离态Spd和Spm的合成。
综上所述，外源Spd可通过下调盐胁迫下adc、odc基因的表达，抑制游离态Put的积累，上调samdc基因的表达促进游离态Spd和Spm的积累，进而缓解盐胁迫对植物生长的抑制。
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